Abstract The role of adipocytes in melanoma The successful growth of primary tumors and establishment of metastases relies heavily on the suitability of the tumor microenvironment. Adipocytes constitute a major portion of the melanoma TME in several cancer types but their contribution to tumor progression is poorly understood. Here, we use an in vivo zebrafish melanoma transplantation model and a co-culture system to investigate the cross-talk between adipocytes and melanoma. In fish, melanoma cells often in subcutaneous sites show a 50% increase in lipid content compared to parental cells. This is accompanied by upregulation of the long chain fatty acid (LCFA) fatty acid transporter proteins FATP2 and FATP6 and down regulation of lipid synthesis genes, suggesting that the increase in lipids is derived from exogenous uptake from the surrounding microenvironment. These data suggest that TME adipocytes can provide lipids to tumor cells. Consistent with this, fish and human melanoma cells co-cultured with 3T3-L1 adipocytes also have increased lipid content, and fluorescently labeled fatty acids can be transferred directly from adipocytes to melanoma cells. Moreover, co-culture with adipocytes makes melanoma cells more invasive and proliferative. Lipid uptake by melanoma cells can be blocked with lipofermata, a small molecule inhibitor of the FATP fatty acid transporters. Strikingly, lipofermata also has a profound anti-proliferative effect on both human and zebrafish melanoma cells. Taken together, these data suggest that TME adipocytes play an important role in melanoma growth, acting as a rich source of lipids for melanoma, and that uptake of these lipids is mediated by the FATP fatty-acid transport proteins. The goal of the proposed research is to test the hypothesis that adipocytes in the microenvironment aid in melanoma progression and metastases. To do so, we aim to elucidate how melanoma cells influence adipocytes and vice versa. First, we will characterize the identify pathways and genes important in mediating increased invasion and proliferation in cocultured tumor cells with gene expression analysis. Moreover, we will determine whether adipocytes mediate metabolic reprogramming in melanoma cells, and test whether these phenotypic changes depend on lipid uptake via FATP proteins. We will also use a transgenic zebrafish line with adipocyte-specific expression of tdTomato-nitroreductase for imaging and isolation of adipocytes after tumor cell transplantation. Fish will imaged live to characterize changes in adipocyte morphology in response to tumor burden. Further, adipocytes will be isolated via FACS to characterize gene expression changes. Finally, adipocytes will be inducibly ablated using the NTR/metronidazole system in the melanoma transplanted fish to functionally test the role of adipocytes in melanoma growth and metastases. Ultimately, we wish to identify clinically targetable pathways of cross-talk between adipocytes and melanoma cells, which represents a novel form of microenvironmental therapy for the treatment of melanoma.